The HP palmtop has several features to extend battery life and maintain system RAM. For a discussion of the features designed to protect system RAM integrity, see "Battery Check" on page 4-142. There are three states that the HP palmtop enters regularly during normal operation:
Most units that are turned on, stay in the light sleep state a good percentage of time. When in this state, the unit will use less power than in the run state. This state is entered from the BIOS Int 15h, Function 41xxh (Wait-For-Event) service. Some of the Int 16h services invoke the Wait-For-Event function and will enter the light sleep state. TIMER0, TIMER1, KBD, and RTC Interrupts are typical events that cause transition from the light sleep to the run state.
When a unit is placed in the deep sleep state it will use significantly less power than one in the run or light sleep states. When the unit is placed in this state the CPU is reset (register contents lost). When "awakened" from this state, execution starts at the reset vector. Under normal circumstances the BIOS makes this transparent to the application, and execution is eventually returned to the code that caused the deep sleep state to be entered. The deep sleep state may be entered in one of several ways: